The present application relates to the field of radiation imaging. It finds particular application to imaging systems that use photon counting detector arrays to determine detection events and pulse pileups.
Today, imaging systems that use radiation to image an article, such as computed tomography (CT) systems, single-photon emission computed tomography (SPECT) systems, digital projection systems, and/or line-scan systems, for example, are useful to provide information, or images, of interior aspects of an article under examination. Generally, the article is exposed to radiation comprising photons (e.g., x-rays, gamma rays, etc.), and an image(s) is formed based upon the radiation absorbed and/or attenuated by interior aspects of the article, or rather an amount of radiation photons that is able to pass through the article. Generally, highly dense aspects of the article absorb and/or attenuate more radiation than less dense aspects, and thus an aspect having a higher density, such as a bone or metal, for example, may be apparent when surrounded by less dense aspects, such as muscle or clothing.
Imaging systems typically comprise a detector array having one or more detector cells. Respective detector cells are configured to indirectly or directly convert radiation photons impingent thereon into electrical charge which is used to generate an electrical signal. The detector cells are typically “energy integrating” or “photon counting” type detector cells (e.g., the imaging system operates in energy integrating mode or photon counting mode).
Energy integrating detector cells are configured to integrate the electrical charge generated over a period of time (e.g., at times referred to as a measurement interval or view) to generate a signal that is proportional to an incoming radiation photon flux rate at a detector cell. While energy integrating detector cells are widely used, there are several drawbacks to this type of cell. For example, energy integrating detectors cells are generally not able to provide feedback as to the number and/or energy of radiation photons detected. As another drawback, there is a lower limit of detection defined by noise such that a detector cell with little to no incident radiation may produce some signal due to thermal and/or analog read noise (e.g., produced by a radiation detection element and/or electronics arrangement of the detector cell). It may be appreciated that as a result of this lower limit, the dose of radiation that is applied to an object under examination is generally greater than the dose of radiation that may be applied to the object if the detector cells are of a photon counting type.
Photon counting type detector cells are configured to output a signal (e.g., a pulse) for respective detected radiation photons (e.g., where the detection of a radiation photon may be referred to as a detection event). In some embodiments, the signal (e.g., or an amplitude of the pulse) is indicative of a radiation energy of the detected radiation photon. A controller is configured to determine the location and energy of respective detected radiation photons based upon the pulse, accumulate the detection events occurring during a measurement interval, digitize the information, and/or process the digital information to form an image, for example.
One drawback of photon counting type detector cells relates to a phenomenon known as pulse pileup. Pulse pileup occurs when two or more photons strike a detector cell in close temporal proximity, causing the pulse of the first photon strike to be combined with the pulse of a second photon strike because the first pulse does not have time to decay before the second photon strike. Thus, a pulse generated from the second photon strike effectively extends the first pulse. Because the pulse of the second photon strike is combined with the pulse of the first photon strike, the system may mistake the detection event as a single photon strike while not recognizing the second photon strike, for example.